Add version support in response header

Added the ability to specify a response header key in the configuration. When defined, a header field with that key and a value of "true" or "false" (depending on whether the requested version is supported or not) will be set on the response.

Expose is_older_version and is_newer_version booleans to allow someone to customize their error handling depending on whether the requested version is deprecated or newer than any of the supported versions.